1. Field of the Invention
The present invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structures for such bits.
2. Background of the Technology
An earth-boring drill bit is coupled to the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string (i.e., weight-on-bit or WOB), the rotating drill bit engages the formation and forms a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process has a diameter generally equal to the diameter or “gage” of the drill bit.
Earth boring bits used in oilfield drilling operations are frequently one of two types: fixed cutter bits or rolling cutter bits. Fixed cutter drill bits have multiple cutting surfaces that are pressed into and dragged through a formation. This type of bit primarily cuts the formation by shearing and scraping. Rolling cutter bits include one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones or rolling cone cutters. The earth disintegrating action of rolling cutter bits is enhanced by providing a plurality of cutters or cutting elements that extend from each of the rolling cones. Applying weight to the drill bit while rotating forces the cutting elements into engagement with the earth and rotates the cones. A rolling cutter drill bit primarily cuts the formation by compression, crushing, gouging, chipping and scraping. Two common classifications of rolling cutter drill bits include “insert” bits and “tooth” bits. In insert bits, the cutting elements extending from the cones comprise inserts that are press fit into undersized apertures in the cone surface prior to drilling with the bit. In tooth bits, the cutting elements comprise teeth that are milled, cast or otherwise integrally formed with the rolling cone.
While drilling, it is conventional practice to pump drilling fluid (also referred to as “drilling mud”) down the length of the tubular drill string where it is jetted from the face of the drill bit through nozzles. The hydraulic energy thus supplied flushes the drilled cuttings away from the cutters and the borehole bottom, and carries them to the surface through the annulus that exists between the tubular drill string and the borehole wall.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section-by-section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section-by-section.
As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Because drilling costs are typically thousands of dollars per hour, it is desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardnesses. The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (ROP), as well as its durability or ability to maintain an acceptable ROP. For the foregoing reasons, it is desirable for the cutting elements of a rolling cone bit to be of a hard, strong, and durable material capable of drilling through hard and/or soft formations without rapid wear.
The shape and positioning of the cutting elements (both teeth and inserts) also impact bit durability and rate of penetration (ROP) and thus, are important to the success of a particular bit design. Cutting elements may have many different shapes, but are commonly chisel or conical in shape. When rolling cutters engage a formation under pressure, cracks develop in the formation and rock fragments and chips may become dislodged. As the cone rotates, the cutting elements penetrate the formation forming a crush zone beneath the tip of each cutter element. As each cutter element penetrates further into the formation, cracks may be formed around the crater created by the cutter element. Chisel shaped cutters commonly form a pair of hertzian cracks at each end of the crest that lead to chip formation. The size of the chips formed while drilling is generally related to the ROP of the drill bit.
During operation, cutting elements undergo large stress fluctuations due to the rotation of the rolling cutters. Large stresses and large stress fluctuations may cause cutting elements to break. As cutting elements penetrate the formation, the stresses typically increase. When cracks form in the formation, some cutter element stress is relieved immediately as the cutter element penetrates further into the formation. Large stress fluctuations also have an effect on the bit bearings positioned between each roller cone and a journal extending from the bit body, and can negatively impact bit bearing operational life.
Accordingly, there remains a need in the art for a drill bits and associated cutting elements that provide a relatively high rate-of-penetration and footage drilled, while at the same time, minimize the effects of wear and the tendency for breakage. Such bits would be particularly well received if they enhanced formation chip size and removal, while minimizing stresses imposed on the cutting elements and bearings.